Not applicable.
Not Applicable.
The invention relates to the measurement of short chain branching in an ethylene 1-olefin copolymer as a function of its molecular weight. More particularly, the invention relates to such a measurement carried out by combining size exclusion chromatography, infrared (such as Fourier transform-infraredxe2x80x94xe2x80x9cFT-IRxe2x80x9d) spectrophotometry, and chemometric analysis.
One property of synthetic polymers, such as olefin copolymers, is that
these macromolecules have a molecular weight distributionxe2x80x94some of the polymer chains are longer than others.
An olefin copolymer also can be characterized by its degree of short-chain branching. The degree of short-chain branching can be determined by determining the number of methyl groups per 1000 carbon atoms in the sample. Given the average molecular weight of the sample, the number of methyl groups attributable to the ends of the polymer backbones can be calculated and subtracted from the number of methyl groups per 1000 carbon atoms to determine the number of methyl groups resulting from side branching. Each n-alkyl side chain has one methyl group.
In addition, an olefin copolymer can be characterized by the degree of short-chain branching as a function of its molecular weight distribution. In other words, a polymer can be characterized according to how many side chains are present on low-molecular weight polymer chains versus high-molecular weight polymer chains in a single bulk copolymer sample.
Information about the degree of short-chain branching (xe2x80x9cSCBxe2x80x9d) of an olefin copolymer, expressed as a function of the molecular weight distribution (xe2x80x9cMWDxe2x80x9d) of the copolymer, is useful for optimizing various properties of the olefin copolymer. Short-chain branching of the copolymer as a function of its molecular weight distribution affects such properties as the density, solvent extractables, and stress crack resistance of olefin copolymers. With this short-chain branching information in hand, the resin designer can modify the olefin copolymer polymerization process to optimize these properties of the resulting copolymer product.
The conventional analysis of short chain branching in an ethylene 1-olefin copolymer as a function of its molecular weight distribution involves solvent fractionation and subsequent characterization by NMR spectroscopy. Although the resulting values for short-chain branching distribution (xe2x80x9cSCBDxe2x80x9d) are highly accurate, gathering the wanted information is labor and time intensive.
Chemometric analysis is a multivariate statistical technique of mathematically treating data from a plurality of measurements to improve the selectivity of the analytical results. See, for example, Stetter, J. R., Jurs, P. C., and Rose, S. L., Anal. Chem. Vol. 58, pp. 860-866 (1986), cited in U.S. Pat. No. 4,874,500. Also, see the text Chemometrics-a Practical Guide, by K. R. Beebe, R. J. Pell, and M. B. Seasholtz, Wiley, N.Y., 1998.
The inventors are not aware that chemometric analysis has been used to assist the determination of the degree of short-chain branching in a sample as a function of its molecular weight.
U.S. Pat. No. 5,700,895 (the ""895 patent) discloses a method to measure the coefficient of variation of chemical composition distribution, Cx, and claims an ethylene-xcex1-olefin copolymer having Cx of 0.40 to 0.80 among five parameter limitations. The method (column 12, lines 30-67 and column 13, lines 1-37) includes FT-IR measurement of temperature rising elution fractions (xe2x80x9cTREFxe2x80x9d fractions) at each of 39 temperatures in the range xe2x88x9210 to 145xc2x0 C. Chemical composition distribution, i.e. short chain branching obtained from spectral peak areas over the interval from 2983 to 2816 cmxe2x88x921 (SCBi), is plotted as a function of elution temperature.
The ""895 patent does not disclose using size-exclusion chromatography (xe2x80x9cSECxe2x80x9d) for fractionation or chemometric analysis for comparison of FT-IR curves. A TREF analysis involves the separation of a sample into fractions based on their differences in solubility in a solvent at different temperatures. Since both the molecular weight of a fraction and its degree of branching have impacts on its solubility, this technique does not separate the respective contributions of these two factors. This technique thus does not allow one to determine the degree of branching as a function of molecular weight. This technique also does not provide information on the statistical error of the results. This technique is also laborious.
U.S. Pat. No. 5,039,614 (the ""614 patent) discloses a method to form solute films from solutions originating from fractionation based on a combination of physical and chemical property differences of ethylene/propylene copolymers (see claims 1 and 6 of the ""614 patent). The films are obtained by rapid evaporation of solvent fractions from a gel permeation chromatography (xe2x80x9cGPCxe2x80x9d) column. FT-IR data on the films characterizes the composition distribution of each polymer fraction. This is exemplified for two ethylene/propylene copolymer resin (xe2x80x9cEPRxe2x80x9d) samples (column 8, lines 19-20). Chemometric analysis is neither suggested nor disclosed. The ""614 patent discloses the formation of a polymer film by solvent evaporation before use of FT-IR to measure co-monomer incorporation.
U.S. Pat. No. 5,151,474 discloses and claims an ethylene polymerization process control method that uses FT-IR or other methods (column 4, lines 3-51) and chemometric analysis (column 4, lines 52-57) to measure the proportion of ethylene and 1-octene in a heptane solvent. There is no suggestion of polymer fractionation or a branching measurement.
Blitz, J. P. and McFaddin, D. C., xe2x80x9cCharacterization of Short Chain Branching in Polyethylene Using Fourier Transform Infrared Spectroscopyxe2x80x9d, J. APPL. POLYM. SCI. 1994, 51, 13-20, discloses the use of methyl and methylene rocking bands in the infrared spectrum to distinguish and quantify methyl, ethyl, butyl, hexyl and isobutyl branches in linear low-density polyethylene (xe2x80x9cLLDPExe2x80x9d). There is no suggestion of polymer fractionation.
Eric T. Hsieh, Chung C. Tso, Jim Dyers, Timothy W. Johnson, Qiang Fu, and Stephen Z. D. Cheng, xe2x80x9cIntermolecular Structural Homogeneity of Metallocene Polyethylene Copolymers,xe2x80x9d J. MACROMOL. SCI.-PHYS. B36(5), 615-628 (1997) discloses measurement of SCB distribution of polymer blends using cross fractionation and 13C NMR (carbon-13 nuclear magnetic resonance).
The conventional methods for fractionating polyolefins are laborious and time-consuming. For example, a single typical cross-fractionation analysis may require 40 to 50 different samples to be processed. Because each sample requires a minimum of 24 hours to process, just the separation step alone requires at least 40 days. Furthermore, an additional 24 hours is needed to analyze each sample by NMR, thereby requiring another 40 days to complete the analysis. The cross-fractionation technique also has the disadvantage of not providing a determination of the statistical error arising from the analysis, as a function of polymer chain length.
One object of the invention is to obtain short-chain branching information about a sample as a function of its molecular weight distribution.
Another object of the invention is to provide short-chain branching distribution information about a sample in a relatively short time, so the information can be collected, analyzed, and used to control the process represented by the sample more quickly and less expensively.
An additional object of the invention is to provide short-chain branching information by a highly mechanized method that directly feeds a sample to an integrated analytical machine.
Still another object of the invention is to provide additional and more timely information about the relation of short-chain branching to molecular weight in polyolefins that are being produced. This information allows a resin designer to adjust the resin density and processing properties to desired values.
Yet another object of the invention is a method of determining the magnitude of the statistical error in the degree of short-chain branching, as a function of the molecular weight distribution. In other words, one can separately determine the amount of random error to be assigned to short-chain branching data at a given molecular weight.
One or more of the preceding objects, or one or more other objects which will become plain upon consideration of the present specification, are satisfied in whole or in part by the invention described herein.
One aspect of the invention, which satisfies one or more of the above objects in whole or in part, is a method of determining the short-chain branching distribution in a hydrocarbon sample.
At least two hydrocarbon training samples having different, known degrees of short-chain branching are provided. Infrared (such as FT-IR) absorbance spectra of the training samples are obtained. The spectra are examined to find at least one parameter that correlates with the known difference in the degree of short-chain branching among the training samples. The parameter can be found, for example, by chemometric analysis. Chemometric analysis is used to define a mathematical relationship between the value of the selected parameter and the degree of short-chain branching in the training samples.
A hydrocarbon test sample requiring analysis is provided. The values of the parameter found by analysis of the training samples are measured for the test sample. The mathematical relationship found by analysis of the training samples is applied to these parameter values for the test sample. As a result, the degree of short-chain branching in the test sample is determined.
The test sample is optionally treated to isolate at least one fraction having a particular molecular weight range (and optionally more fractions having different molecular weight ranges). The molecular weight range and the degree of short-chain branching in the fraction can then be determined.
Another aspect of the invention is a method of determining the short-chain branching distribution in a hydrocarbon sample as a function of its molecular weight distribution. In this method, a mathematical relationship is defined between the infrared (such as FT-IR) absorbance values of a test sample at 3000 to 2870 cmxe2x88x921 and the number of methyl groups per 1000 carbon atoms in the olefin copolymer sample.
An olefin copolymer test sample is isolated into at least one fraction having a particular molecular weight range. The infrared spectrum of the fraction is measured from about 3000 to about 2870 cmxe2x88x921 for the fraction. The spectral data may also be preprocessed using a variety of mathematical algorithms such as but not limited to data smoothing, baseline corrections, application of derivatives, and mean centering. A further mathematical relationship is then applied to the fraction to determine the degree of short-chain branching in the fraction, using an algorithm typically supplied by the chemometric software. The short-chain branching value given as such is for that particular molecular weight range of which the fraction is composed.
Still another aspect of the invention is a method for determining the statistical error in the measurement of short-chain branching in an olefin polymer sample as a function of its molecular weight distribution.
In this aspect of the invention, multiple replicates of an olefin polymer sample are provided. (xe2x80x9cMultiple,xe2x80x9d in this context, means enough replicates to allow the chosen statistical analysis to be done. As a general rule, the more replicates are provided and analyzed, the more accurate the statistical analysis will be.)
Spectra of the replicate sample fractions are obtained in a wavenumber range useful for determining their degrees of short-chain branching. The degrees of short-chain branching in the replicate sample fractions are determined by analysis of the spectra. The statistical errors in the short-chain branching results in the replicate sample fractions are determined, as a function of molecular weight. The areas of the spectra of the replicate sample fractions, as a function of molecular weight, are also determined.
The statistical errors and areas for the respective fractions are then correlated by finding values of the slope m and intercept b in the following equation that at least approximate the relation between the statistical error and the area of the spectra for the respective replicate sample fractions:
E=mAxe2x88x92xc2xd+b 
In this equation, E is the statistical error for a sample fraction having a particular molecular weight, A is the area of the spectrum of the sample fraction, and m is the slope and b is the intercept. This equation can be solved by the chemometric method of partial least squares analysis.
A significant advantage of the present invention is that an analysis of the degree of short-chain branching in a sample can be carried out relatively quickly (in minutes) and with far less effort than before. This advantage allows the degree of short-chain branching in polymerization products, as a function of molecular weight, to be measured many times daily in the ordinary course of production, if desired. As with any process, it is advantageous to be able to provide timely, frequent information about the status of the reaction or other processing of an olefin copolymer process stream so the process conditions can be maintained within more precise specifications than before. Also, short-chain branching information can be gathered at lower cost than before. In addition, the analysis can be more statistically sophisticated than before. The statistical error can be determined as a function of the molecular weight distribution of the sample.
As a result, more accurate information can be obtained respecting whether a difference between two analyses is statistically significant, without the need to do replicate analyses and calculate the degree of statistical error each time the analysis is performed.